An elastic spring of unstretched length L and force constant k is stretched by a small length x. It is further stretched by another small length y. The work done in the second stretching is

Potential energy stored in the spring when it is extended by x is U1=12kx2 Potential energy stored in the spring when it is further extended by y is U2=12k(x+y)2 ∴  work done = U2−U1=12k(x+y)2−12kx2=12ky(2x+y)